1. Field of the Invention
The present invention relates to an apparatus for detecting the information on the displacement of a moving object or a fluid (hereinafter referred to as a moving object), or a velocimeter that measures the speed of a moving object, without contact. More particularly, it relates to a Doppler velocimeter that detects the shift of a frequency of illuminating light.
2. Related Background Art
As an apparatus that measures the movement speed of a moving object without contact and with a high degree of accuracy, a Doppler velocimeter has previously been known. The Doppler velocimeter applies illuminating light, such as a laser beam or the like, to a moving object and measures the movement velocity of the above moving object by utilizing the effect whereby the frequency of scattered light from the moving object shifts proportionally to the movement velocity, the so-called Doppler effect.
A typical construction of a laser Doppler velocimeter is shown in FIG. 1 by way of an example of a conventional Doppler velocimeter.
In FIG. 1, reference numeral 1 denotes a laser beam source; reference numeral 2 denotes a collimator lens; reference numeral 3 denotes parallel light fluxes; reference numeral 4 denotes a beam splitter; reference numerals 6 and 6' each denote a mirror; reference numeral 7 denotes a moving object, which object or fluid moves in the direction of the arrow at a velocity V; reference numeral 8 denotes a condenser lens; and reference numeral 9 denotes a photodetector.
A laser beam emitted from the laser beam source 1 is made into the parallel light fluxes 3 by means of the collimator lens 2. The laser beam is then divided into two light fluxes 5 and 5' by means of the beam splitter 4. After the laser beam is reflected by the mirrors 6 and 6', the two light fluxes are projected to a moving object 7 at an incident angle .theta. at a velocity V. Scattered light from the moving object or fluid is detected by the photodetector 9 via the condenser lens 8. The frequency of the scattered light from the two light fluxes undergoes a Doppler shift of +.DELTA.f and -.DELTA.f, respectively, in proportion to the movement velocity V. If the wavelength of the laser beam is made to be .lambda., f can be expressed by the following equation (1): EQU .DELTA.f=V sin .theta./.lambda. (1)
The scattered light which undergoes the Doppler shift of +.DELTA.f or -.DELTA.f interferes with each other, causing changes in light and darkness on the light-receiving surface of the photodetector 9. The frequency F of the scattered light is given by the following equation (2): EQU F=2f=2V sin .theta./.lambda. (2)
Therefore, the measurement of the frequency (hereinafter referred to as a Doppler frequency) of output signals of the photodetector 9 enables the velocity V of the moving object 7 to be determined on the basis of equation (2).
In the laser Doppler velocimeter of the prior art as described above, as can be seen from equation (2), the Doppler frequency F is inversely proportionaI to the wavelength .lambda. of the laser. Therefore, it is required that a laser beam source, the laser of which is stable, be used for a laser Doppler velocimeter. As a laser beam source in which continuous oscillation is possible and the laser of which is stable, a gas laser using He-Ne or the like is often used. However, its laser oscillator is large and a high voltage is required for the power supply. This presents the problem that the apparatus is large and expensive. Although laser diodes (or semiconductor lasers) used in compact discs, video discs, optical fiber communication, and so forth are ultrasmall and can be easily driven, they are dependent on temperatures.
FIG. 2 (quoted from Photosemiconductor Elements Part, "'87 Mitsubishi Semiconductor Data Book") shows an example of standard temperature dependency of laser diodes. The section where the wavelength changes continuously is mainly caused by changes in the refractive index of the active layers of laser diodes due to temperature, which is 0.05 to 0.06 nm/.degree.C. The section where the wavelength is changed uncontinuously is called vertical mode hopping, which is 0.2 to 0.3 nm/.degree.C.
To stabilize the wavelength, a method of controlling laser diodes at a constant temperature is generally used. In this method, temperature control members, such as heaters, radiators, temperature sensors, or the like, are mounted on laser diodes with a small resistance to heat, and it is required that temperature control be performed precisely. Laser Doppler velocimeter are relatively large, and costs will rise. Furthermore, instability due to the above-mentioned vertical mode hopping cannot be completely eliminated.
Therefore, the present applicant proposed in U.S. Patent application Ser. No. 501,499 a laser Doppler velocimeter which solves the above-mentioned problems and which comprises the steps of applying a laser beam as a light source to diffraction grating, applying two diffracted lights of the +n-th order and -n-th order (n:1, 2, . . . among diffracted lights thus obtained other than that of the zero-order to a moving object at an intersection angle which is the same as the angle made by the two light fluxes, and detecting the scattered light from the moving object by using a photodetector. This method will now be explained.
FIG. 3 shows an example of diffraction when a laser beam I is projected onto a transmission type diffraction grating 10 having a grating pitch d to be perpendicular to the direction t in which the grating is arrayed. In such a case, diffraction angle .theta..sub.0 becomes as shown in the following equation: EQU sin .theta..sub.0 /.lambda.=m.lambda./d
where m:diffraction order (0, 1, 2, . . .), .lambda.:wavelength of the beam.
.+-.n-th order beams other than the zero-order beam are expressed by the following equation (3): EQU sin .theta..sub.0 =.+-.n.lambda./d (3)
where n:1, 2, . . . .
FIG. 4 is a view showing a case in which two light fluxes are applied to a specimen object 7 by means of mirrors 6 and 6' in such a way that an incident angle becomes .theta..sub.0. The Doppler frequency F of the photodetector 9 is expressed by the following equation (4) by using equations (2) and (3): EQU F=2 V sin .theta..sub.0 /.lambda.=2 n V/d (4)
Thus, the Doppler frequency F does not depend on the wavelength .lambda. of the laser beam I. This frequency is inversely proportional to the grating pitch d of the diffraction grating 10 and proportional to the speed of the specimen object 7. Since the grating pitch d can be made satisfactorily stable, a frequency which is proportional to only the speed of the moving object 7 can be obtained for the Doppler frequency F. The same is true even when a reflection type diffraction grating is used for the diffraction grating 10. As described above, an optical system is formed in which sin .theta..sub.0 /.lambda. having an incident angle which is made to be .theta..sub.0 becomes constant. Thus, a high-precision measurement which is not dependent on a wavelength can be realized.
If light having a generally high coherency, such as a laser beam or the like, is applied to an object, scattered light undergoes a random phase modulation due to very small irregularities on the surface of the object, and a speckled pattern is formed on the observation surface. When an object or a fluid moves in the laser Doppler velocimeter, changes in light and darkness due to the Doppler shift are modulated by irregular changes in light and darkness due to the flow of the speckled pattern on the detection surface of the photodetector. Also, the output signal of the photodetector is modulated by changes in the transmittance (or reflectance) of the specimen object.
In the above-mentioned G-LDv, the frequency of changes in light and darkness due to the flow of the speckled pattern and the frequency of changes in the transmittance (or reflectance) of a specimen object are generally lower than the Doppler frequency expressed by the above equation (4). Therefore, a method is used in which low-frequency components are electrically eliminated by making the output of the photodetector pass through a high-pass filter so that only Doppler signals are extracted. If the speed of the specimen object is slow and the Doppler frequency is low, the frequency difference between the Doppler frequency and low-frequency components becomes small. In order to deal with a case where a high-pass filter cannot be used or it is desired to detect the direction of the speed, the present applicant proposed in United States Patent application Ser. No. 501,499 the apparatus shown in FIG. 5.
A diffraction grating having a grating pitch d is moved at a velocity Vg, as shown in FIG. 5. When a laser beam is projected onto the moving diffraction grating, the laser beam is divided into .+-.n-th order diffracted lights 5a and 5b. Each of the lights undergoes positive and negative Doppler shifts .+-.Vg/nd, respectively. If the wavelength of the light is made to be .lambda., the diffraction angle .theta..sub.0 satisfies the following condition: EQU sin .theta..sub.0 =.lambda./n d (5)
When these two light fluxes are made incident on the specimen object 7 at a velocity V by means of mirrors 6 and 6' using these .+-.n-th order lights in such a way that the incident angle becomes .theta..sub.0, concerning the scattered light from the specimen object 7, +n-th order light undergoes a Doppler shift of (Vg+V)/nd and -n-th order light undergoes a Doppler shift of -(Vg+V)/nd. These lights interfer with each other, and the Doppler frequency F becomes the following: EQU F=2 (Vg+V)/nd (6)
Thus, a Doppler frequency which is not dependent on the wavelength of the laser beam can be obtained. That is, even if the velocity of the specimen object 7 is slow, the frequency difference between the Doppler frequency and low-frequency components resulting from the flow of the speckled pattern or from the changes in the transmittance (or reflectance) of the specimen object can be satisfactorily extracted by the movement velocity Vg of the diffraction grating. Velocity detection is made possible by a method in which low-frequency components are electrically eliminated by making the output of the photodetector pass through a high-pass filter so that only Doppler signals are extracted.
FIG. 6 shows the relationship between the velocity V of a specimen object in a laser Doppler velocimeter which uses diffraction grating and the Doppler frequency F. FIG. 6A shows a case where the diffraction grating is fixed; FIG. 6B shows a case where the movement speed of the diffraction grating is made to be Vg. As can be seen from these figures, in FIG. 6A, even if a certain frequency F; is detected, it is impossible to judge the direction of the movement of the diffraction grafting because the two velocities V.sub.1 and -V.sub.1 whose directions differ from each other match. However, in FIG. 6B, the Doppler frequency F=Fg+F.sub.1 can be obtained for the velocity V.sub.1, and the Doppler frequency F=Fg-F.sub.1 can be obtained for the velocity -V.sub.1. The direction of the velocity V can also be detected.
That is, if the movement velocity Vg of the diffraction grating is controlled, the following relation holds by using the above-mentioned equation (6): EQU V=F (d/2)-Vg (7)
Therefore, the detection of F enables V to be determined, as expressed by equation (7).